الفهرس 
Title : Quantum modeling of proton exchange of H-NMR
ContentOnly 14 pages are availabe for public view
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Abstract
In this work, we present density functional theory calculations to predict nuclear magnetic resonance (NMR) parameters calculated with gauge Independent atomic orbital (GIAO) for glycine, L-alanine, L-cysteine and cysteine-HCl in DMSO as solvent using IEFPCM model of solvation and B3PW91 functional with both 6-11++G (2d, 2p) and cc-pVDZ basis sets to investigate the changes in NMR parameters during the process of direct proton transfer from neutral form to zwitterion form passing through transition state without the explicit assistance of a solvent molecule. The NMR parameters have been calculated along the reaction paths of the proton transfers by means of the intrinsic reaction coordinate (IRC) procedure. Among the proton shielding constants, the greatest changes are observed for the exchanging protons. Upon migration, the protons first become deshielded and then shielded again, with the strongest deshielding occurring around the transition. The isotropic shielding constant shows a strong deshielding of the oxygen nucleus as the proton migrates away. The one-bond couplings between proton, oxygen and nitrogen dominated by the Fermi-contact term, decay steeply as the internuclear distance increases. In addition, we have tested the performance of our computational approach against experimental data with RMSD 1.37 and 1.27 for both 6-11++G (2d, 2p) and cc-pVDZ basis sets respectively. The vibrational spectra associated with proton transfer along IRC have been calculated first to check both the transition state and stable minimum and to confirm NMR parameters and in agreement with previously reported data